Mycorrhizal fungi provide plants with a range of benefits, including mineral nutrients and protection from stress and pathogens. Here we synthesize current information about how the presence and type ...of mycorrhizal association affect plant communities. We argue that mycorrhizal fungi regulate seedling establishment and species coexistence through stabilizing and equalizing mechanisms such as soil nutrient partitioning, feedback to soil antagonists, differential mycorrhizal benefits, and nutrient trade. Mycorrhizal fungi have strong effects on plant population and community biology, with mycorrhizal type-specific effects on seed dispersal, seedling establishment, and soil niche differentiation, as well as interspecific and intraspecific competition and hence plant diversity.
Co‐existence theories fail to adequately explain observed community patterns (diversity and composition) because they mainly address local extinctions. For a more complete understanding, the regional ...processes responsible for species formation and geographic dispersal should also be considered. The species pool concept holds that local variation in community patterns is dependent primarily on the availability of species, which is driven by historical diversification and dispersal at continental and landscape scales. However, empirical evidence of historical effects is limited. This slow progress can be attributed to methodological difficulties in determining the characteristics of historical species pools and how they contributed to diversity patterns in contemporary landscapes. A role of landscape‐scale dispersal limitation in determining local community patterns has been demonstrated by numerous seed addition experiments. However, disentangling general patterns of dispersal limitation in communities still requires attention. Distinguishing habitat‐specific species pools can help to meet both applied and theoretical challenges. In conservation biology, the use of absolute richness may be uninformative because the size of species pools varies between habitats. For characterizing the dynamic state of individual communities, biodiversity relative to species pools provides a balanced way of assessing communities in different habitats. Information about species pools may also be useful when studying community assembly rules, because it enables a possible mechanism of trait convergence (habitat filtering) to be explicitly assessed. Empirical study of the role of historic effects and dispersal on local community patterns has often been restricted due to methodological difficulties in determining habitat‐specific species pools. However, accumulating distributional, ecological and phylogenetic information, as well as use of appropriate model systems (e.g. archipelagos with known biogeographic histories) will allow the species pool concept to be applied effectively in future research.
More than a decade ago, the Driver (arbuscular mycorrhizal fungal partners drive plant communities) and Passenger (AMF community dynamics follows changes in the host plant community) hypotheses were ...suggested to explain the mutual relationships of plant and AMF communities. We propose one more hypothesis: the Habitat hypothesis, which postulates that both plant and AMF communities follow changes in abiotic conditions. The null hypothesis for all three working hypotheses can be called the Independence hypothesis, which proposes that plant and AMF communities are unrelated. We investigate the assumptions of these hypotheses and the available evidence in support of them. We suggest that community dynamics during secondary succession, including those related to land‐use changes, may be explained by the Driver hypothesis, while the dynamics of plant and AMF communities during primary succession may be explained by the Passenger hypothesis. Within‐region co‐variation of successionally stable plant and AMF communities may be explained by the Habitat hypothesis, while the Independence hypothesis may explain global patterns of plant and AMF communities. These suggestions are tentative, and more evidence from both descriptive and experimental studies is required to assess them. In particular, comparative information is needed about dispersal limitation of plant and AMF communities in dynamic landscapes.
Plant traits determine how individual plants cope with heterogeneous environments. Despite large variability in individual traits, trait coordination and trade-offs
result in some trait combinations ...being much more widespread than others, as revealed in the global spectrum of plant form and function (GSPFF
) and the root economics space (RES
) for aboveground and fine-root traits, respectively. Here we combine the traits that define both functional spaces. Our analysis confirms the major trends of the GSPFF and shows that the RES captures additional information. The four dimensions needed to explain the non-redundant information in the dataset can be summarized in an aboveground and a fine-root plane, corresponding to the GSPFF and the RES, respectively. Both planes display high levels of species aggregation, but the differentiation among growth forms, families and biomes is lower on the fine-root plane, which does not include any size-related trait, than on the aboveground plane. As a result, many species with similar fine-root syndromes display contrasting aboveground traits. This highlights the importance of including belowground organs to the GSPFF when exploring the interplay between different natural selection pressures and whole-plant trait integration.
Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community ...assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co‐occur randomly but are restricted in their co‐occurrence by interspecific competition. This concept can be redefined in a more general framework where the co‐occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive.
Here we present a survey and meta‐analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co‐occurrence, niche limitation, guild proportionality and limiting similarity.
Results from our meta‐analyses suggest that non‐random co‐occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata.
We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non‐random aspect of assembly in plant communities.
Functional trait differences among species are increasingly used to infer the effects of biotic and abiotic processes on species coexistence. Commonly, the trait diversity observed within communities ...is compared to patterns simulated in randomly generated communities based on sampling within a region. The resulting patterns of trait convergence and divergence are assumed to reveal abiotic and biotic processes, respectively. However, biotic processes such as competition can produce both trait divergence and convergence, through either excluding similar species (niche differences, divergence) or excluding dissimilar species (weaker competitor exclusion, convergence). Hence, separating biotic and abiotic processes that can produce identical patterns of trait diversity, or even patterns that neutralize each other, is not feasible with previous methods. We propose an operational framework in which the functional trait dissimilarity within communities (FDcomm) is compared to the corresponding trait dissimilarity expected from the species pool (i.e., functional species pool diversity, FDpool). FDpool includes the set of potential species for a site delimited by the operating environmental and dispersal limitation filters. By applying these filters, the resulting pattern of trait diversity is consistent with biotic processes, i.e., trait divergence (FDcomm > FDpool) indicates niche differentiation, while trait convergence (FDcomm < FDpool) indicates weaker competitor exclusion. To illustrate this framework, with its potential application and constraints, we analyzed both simulated and field data. The functional species pool framework more consistently detected the simulated trait diversity patterns than previous approaches. In the field, using data from plant communities of typical Northern European habitats in Estonia, we found that both niche-based and weaker competitor exclusion influenced community assembly, depending on the traits and community considered. In both simulated and field data, we demonstrated that only by estimating the species pool of a site is it possible to differentiate the patterns of trait dissimilarity produced by operating biotic processes. The framework, which can be applied with both functional and phylogenetic diversity, enables a reinterpretation of community assembly processes. Solving the challenge of defining an appropriate reference species pool for a site can provide a better understanding of community assembly.
Mycorrhizal inoculation can enhance outcomes of ecological restoration, but the benefits may be context-dependent. Here, we performed a meta-analysis of field studies to elucidate conditions in which ...adding mycorrhizal fungi enhances restoration success. We found inoculation increased plant biomass by an average effect size of 1.7 in 70 independent comparisons from 26 field-based studies, with the largest increases to N-fixing woody plants, C4-grasses and plants growing in soils with low plant-available P. Growth responses to inoculation increased with time for the first 3 yr after inoculation, especially for N-fixing woody plants and plants growing in severely altered soils. We found that mycorrhizal inoculation increased species richness of restored plant communities by 30%, promoted establishment of target species, and enhanced similarity of restored to reference communities. We conclude that the addition of mycorrhizal fungi to restoration sites can facilitate rapid establishment of vegetation cover, and restoration of diverse plant communities more akin to reference sites.
An increasing number of case studies are reporting Glomeromycota molecular diversity from ecosystems worldwide. Typically, phylogroups that can be related to morphospecies and those that remain ...unidentified ("environmental") are recorded. To compare such data and generalise observed patterns, the principles underlying sequence identification should be unified. Data from case studies are collected and systematized in a public database MaarjAM (http://www.maarjam.botany.ut.ee), which applies a unique molecular operational taxonomic unit (MOTU) nomenclature: virtual taxa (VT) are phylogenetically defined sequence groups roughly corresponding to species-level taxa. VT are based on type sequences, making them consistent in time, but they also evolve: they can be split or merged, when necessary. This system allows standardisation of original MOTU designations and, much like binomial taxonomic nomenclature, comparison and consistency between studies. Refinement ofVT delimitation principles and comparability with traditional Glomeromycota taxonomy will benefit from more information about intra- vs. inter-specific nucleotide variation in Glomeromycota, sequencing of morphospecies, and resolution of issues in Glomeromycota taxonomy. As the recorded number of VT already exceeds the number of Glomeromycota morphospecies, designation of species based on DNA alone appears a necessity in the near future. Application of VT is becoming widespread, and MaarjAM database is increasingly used as a reference for environmental sequence identification. The current status and future prospects of arbuscular mycorrhizal fungi (AMF) DNA-based identification and community description are presented.
Native biodiversity decline and non-native species spread are major features of the Anthropocene. Both processes can drive biotic homogenization by reducing trait and phylogenetic differences in ...species assemblages between regions, thus diminishing the regional distinctiveness of biotas and likely have negative impacts on key ecosystem functions. However, a global assessment of this phenomenon is lacking. Here, using a dataset of >200,000 plant species, we demonstrate widespread and temporal decreases in species and phylogenetic turnover across grain sizes and spatial extents. The extent of homogenization within major biomes is pronounced and is overwhelmingly explained by non-native species naturalizations. Asia and North America are major sources of non-native species; however, the species they export tend to be phylogenetically close to recipient floras. Australia, the Pacific and Europe, in contrast, contribute fewer species to the global pool of non-natives, but represent a disproportionate amount of phylogenetic diversity. The timeline of most naturalisations coincides with widespread human migration within the last ~500 years, and demonstrates the profound influence humans exert on regional biotas beyond changes in species richness.
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